This invention relates to extrusion apparatus for melting solid plastic and generating hydrostatic pressure to force the melt through a die, and more particularly to an improved screw for use in such apparatus.
Continuously operating screw extruders for processing synthetic resinous plastic material are conventionally gravity fed from a source of stock in solid or particulate form to a screw portion beneath the feed inlet having the same or greater conveying capacity as that beyond the inlet. Systems under such conditions are said to operate in a flood feed mode, and traditionally exhibit a certain level of mass flow surging in the outlet die particularly at high throughput rates which results in corresponding undesirable fluctuations in extrudate weight. Such surging as manifested by fluctuating melt pressure in the delivery tube to the die is believed caused by a fluctuation in the axial position of the solid-melt interface along the screw. More specifically, when such interface moves rearwardly toward the feed end of the system, pressure and therefore mass flow through the die is increased whereas forward movement decreases pressure and mass flow. This fluctuation of the interface is a function of a number of variables such as, for example, extruder barrel temperature profile, head of material in the hopper feeding the extruder, temperature and bulk density of the feed stock, level of buildup of volatile products within the extruder and the like. Likewise characteristic of systems operating under flood feed conditions at high throughput rates are reduced power economy (mass extrustion rate divided by mechanical power) and increased occlusion of air which, if present, results in bubbles in the extrudate.